1. Field of the Invention
The present invention relates to a drive system for a motor vehicle including an electric machine and a drive shaft, the electric machine being a starter/generator for rotating the drive shaft and obtaining electric energy from a rotating drive shaft, the electric machine having a stator arrangement and a rotor arrangement which can be rotated by the drive shaft and is operated by permanent magnet.
2. Description of the Related Art
In modern drive systems, electric machines are frequently used in so-called starter/generator arrangements to start a drive unit such as, for example, an internal combustion engine, and to generate electric energy when the drive unit is running and the vehicle is in operation for feeding the electrical energy directly into the vehicle network or to charge a battery, i.e., store the energy. However, such electric machines are also used for generating drive energy to supplement or as an alternative to an internal combustion engine. In electric machines with a rotor arrangement operated by permanent magnet, the magnetic field produced by the permanent magnets provided in the rotor arrangement induces a back-e.m.f. in the stator which becomes increasingly larger as the speed of the rotor arrangement increases. If this back-e.m.f. becomes greater than an operating voltage which is, or can be applied to the stator arrangement or stator windings, the electric machine enters a weakened field region in which, by suitably feeding current to the stator windings, a portion of the electric current is used for the purpose of electronic field weakening to be able to operate the electric machine further. The fraction of the field weakening current increases with the speed of the machine, thereby lowering the efficiency of the electric machine at high speeds.
The object of the present invention is to provide a drive system which has an improved operating capacity over a large speed range.
According to the present invention, the object is achieved by a drive system for a motor vehicle comprising a drive shaft and an electric machine arranged as a starter/generator unit for rotating the drive shaft and obtaining electric energy from the drive shaft when the drive shaft is rotating. The electric machine has a stator arrangement and a rotor arrangement which can be rotated with the aid of the drive shaft and is operated by permanent magnet.
The drive system according to the present invention further include a heat transfer arrangement which may be used, preferably via a heat transfer fluid, to bring the rotor arrangement to a desired operating temperature and/or to maintain the rotor arrangement within a region comprising the desired operating temperature.
The basis of the present invention is the finding that, particularly in the case of rotor arrangements operated by permanent magnet, the magnetic field produced by the permanent magnets decreases with increasing temperature. According to the present invention, the temperature in the region of the rotor arrangement is set to a prescribed relatively high temperature. This ensures that the magnetic field produced by the permanent magnets is smaller, particularly in the case of high speeds. The consequence of this is that the back-e.m.f. induced by the magnetic field of the permanent magnets in the stator arrangement also decreases. The fraction of the field weakening current also decreases and/or the entry into a weakened field region does not occur until a relatively high speed.
The desired temperature may be set, for example, by a heat transfer arrangement comprising a fluid channel arrangement at a housing region arranged adjacent to the rotor arrangement through which heat transfer fluid can flow.
To avoid damage in the region of the rotor arrangement from high temperatures, it is proposed that the desired operating temperature is set to approximately 10-50xc2x0 C. below a temperature at which an irreversible impairment of the magnetization of the rotor arrangement operated by permanent magnet occurs. For example, the desired operating temperature may be situated in the region of 100xc2x0 C.-200xc2x0 C. and is preferably within the range including approximately 140xc2x0 C.-160xc2x0 C.
The drive system comprises a drive unit which is to be cooled by a cooling fluid. To configure the drive system according to the present invention as simply as possible, the cooling fluid used in the drive unit forms the heat transfer fluid for the heat transfer arrangement.
Drive units which are cooled generally include internal combustion engines which are at first still cold when put into operation. That is, the cooling fluid, which is usually cooling water, is at a prevailing ambient temperature when the ICE is initially started. Immediately following the start of the internal combustion engine, the temperature thereof rises relatively rapidly. Accordingly, the temperature of the cooling fluid in the internal combustion engine also rises at a rate which is quicker than the temperature in the region of the rotor arrangement. To utilize this rise in temperature for bringing the rotor arrangement relatively quickly to the desired operating temperature, in a first operating phase in which the temperature of the heat transfer fluid is higher than the temperature prevailing in the region of the rotor arrangement, and the temperature prevailing in the region of the rotor arrangement is lower than the desired operating temperature, the heat transfer fluid is fed to the rotor arrangement for the purpose of heat transfer. This effect may be used not only when an internal combustion engine to be cooled is used as a drive unit, but always whenever a heat transfer fluid such as, for example, a cooling fluid has a higher temperature than that temperature which prevails in the region of the rotor arrangement.
Internal combustion units typically have a maximum temperature for the cooling fluid in the range of approximately 80xc2x0 C.-120xc2x0 C. However, this range is generally below the temperature at which, in accordance with the present invention, the rotor arrangement preferably operates. Thus, if the procedure of feeding this cooling fluid as heat transfer fluid to the rotor arrangement or to the heat transfer arrangement for the rotor arrangement were to be continued, it would be difficult to achieve the desired operating temperature at the rotor arrangement in a relatively short time. It is therefore proposed that in a second operating phase in which the temperature of the heat transfer fluid is not higher than the temperature prevailing in the region of the rotor arrangement, and the temperature prevailing in the region of the rotor arrangement is lower than the desired operating temperature, the supply of the heat transfer fluid from the drive unit is at least diminished and preferably essentially completely interrupted. In this phase, heat may be fed to the rotor arrangement from other heat sources such as, for example, the internal combustion engine via thermal radiation or from a coupling arrangement or, if appropriate, also from the stator arrangement.
In a third operating phase in which the temperature prevailing in the region of the rotor arrangement is in the region of the desired operating temperature, the heat transfer fluid is fed for the purpose of keeping the rotor arrangement at the desired operating temperature. In effect, this means that heat transfer fluid which transfers thermal energy to the rotor arrangement during the starting phase now extracts from the rotor arrangement thermal energy which is fed, for example, to the stator arrangement, directly to an internal combustion engine or to a friction clutch.
It is preferably provided in the present invention that the desired operating temperature is a function of the speed of the rotor arrangement. The desired operating temperature may be higher during a higher speed of the rotor arrangement than during a lower speed of the rotor arrangement. The invention makes use here of the fact that, as already mentioned, the magnetic flux or the field strength of the permanent magnets of the rotor arrangement generally increases with decreasing temperature. However, at lower speeds the induced back-e.m.f. is still low, in any case lower than the operating voltage applied to the stator arrangement, so that here prescribing a lower desired operating temperature firstly has the advantage that a greater efficiency of the electric machine can be achieved because of the higher field strength of the permanent magnets.
The present invention further relates to a method for operating a drive system, the drive system having a drive shaft and an electric machine connected for rotating the drive shaft and obtaining electric energy from the rotation of the drive shaft, the electric machine having a stator arrangement and a rotor arrangement which can be rotated via the drive shaft and is operated by permanent magnet, the method comprising the steps of:
(a) determining a desired operating temperature for the rotor arrangement; and
(b) setting the temperature prevailing in the region of the rotor arrangement at the desired operating temperature via a closed temperature loop.
Step (b) may comprise setting the temperature prevailing in the region of the rotor arrangement by feeding a heat transfer fluid to the rotor arrangement.
To avoid damaging the electric machine, particularly in the case of defective drives, the desired operating temperature is a temperature which is situated approximately 10-50xc2x0 C. below a temperature at which an irreversible impairment of the magnetization of the rotor arrangement operated by permanent magnet may occur.
Step (b) may comprises the following measures:
(c) in a first operating phase in which the temperature of the heat transfer fluid is higher than the temperature prevailing in the region of the rotor arrangement, and the temperature prevailing in the region of the rotor arrangement is lower than the desired operating temperature, feeding the heat transfer fluid to the rotor arrangement for the effecting heat transfer,
(d) in a second operating phase in which the temperature of the heat transfer fluid is not higher than the temperature prevailing in the region of the rotor arrangement, and the temperature prevailing in the region of the rotor arrangement is lower than the desired operating temperature, diminishing the feeding of the heat transfer fluid and preferably interrupting the feeding of the heat transfer fluid, and
(e) in a third operating phase in which the temperature prevailing in the region of the rotor arrangement is in the region of the desired operating temperature, feeding the heat transfer fluid maintaining the temperature of the rotor arrangement at the desired operating temperature via a closed temperature loop.
As already described previously, the desired temperature for the rotor arrangement may be set in step a) as a function of the speed of the rotor arrangement, wherein the desired temperature increases as the speed increases.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.